Commodity plastics such as polyethylene (PE) and polypropylene (PP) are used in numerous applications ranging from toys, containers and food packaging to prosthetics. Polyolefins are subjected to elevated temperature during conventional melt processing operations such as twin-screw melt extrusion, rotational molding, and thermoforming and are highly susceptible to oxidative degradation. The homolytic cleavage of alkyl hydroperoxides generates free radicals which promote an autocatalytic degradation process. Without an adequate level of thermal stabilizers, polyolefins can undergo oxidative degradation by chain scission and/or chain extension, thereby forming networks and/or low molecular weight fractions. Furthermore, long-term environmental weathering from exposure to ultraviolet radiation and successive melt extrusion associated with recycling can amplify material degradation and deteriorate mechanical performance and optical properties.
Antioxidants are added to polyolefins during processing to improve high-temperature processability, long-term weathering resistance, and recyclability. Sterically hindered phenols are an important class of synthetic antioxidants for polymers. Such hydrogen-donating hindered phenols act as chain-breaking antioxidants that prevent the propagation of free radical reactions by trapping alkyl peroxy radicals. Commercial antioxidants such as Irganox™ I1010 are based on the family of hindered phenols like 4-substituted-2,6-ditertiary butyl phenols. The effectiveness of thermal stabilizers or antioxidants in stabilizing polymers depends on many factors including solubility, dispersion, ability to stabilize different polymer matrices, evaporation or volatilization during processing, conditions of use, and recycling.
However, stabilization of polyolefins with synthetic antioxidants poses potential health issues and environmental challenges. Typical hindered phenols are able migrate to the surfaces of polymeric articles. Plastics that are used for food packaging have come under scrutiny because synthetic antioxidants may contaminate food and serve as a source of unknown cytotoxicity. Few details are known about the toxicity and migration of the transformation products of such antioxidants produced during processing and application. Other environmental concerns arise from the presence of trace levels of heavy metal catalyst used for antioxidant synthesis. Moreover, economic and environmental factors have led to growing interest in using greener, sustainable materials in place of synthetic counterparts as feedstocks for thermoplastics and their composites. Examples range from the use of natural monomers to the fabrication of green polymer composites employing natural or agro-based residues as reinforcements. Such composites provide the benefits of biodegradability, reduced wear to processing equipment, and low cost. In light of these benefits as well as concerns over the use of synthetic antioxidants, numerous bio-based phenols such as caffeic acid, ascorbic acid (vitamin C), α-tocopherol (vitamin E), curcumin, quercetin, and β-carotenes have been incorporated into different packaging materials as thermal stabilizers.
Relating thereto, Al-Malaika et al. examined the efficacy of the commercially-available compound vitamin E as a melt stabilizer for low density polyethylene (LDPE). After four extrusions at 180° C., the unstabilized LDPE control sample showed a 20% decrease in melt flow index (MFI) due to chain branching. In contrast, incorporation of 0.2 wt % vitamin E maintained the viscosity of extruded LDPE nearly constant, resulting in only a 2.5% increase in MFI after four extrusions. Interestingly, the stabilization achieved by incorporating 0.2 wt % vitamin E was better than that by incorporating 0.2 wt % synthetic antioxidants such as Irganox™ I1010 and I1076, both of which led a 7.5% increase in MFI after four extrusions. Tatraaljai et al. similarly investigated the effect of curcumin, found in turmeric and other roots, on the processing stability of PE. Using various characterization techniques, they demonstrated that PE with 0.1 wt % curcumin showed superior thermo-oxidative stability compared to that for PE with 0.1 wt % Irganox™ I1010. They attributed the enhanced stability to the ability of curcumin to arrest alkyl radical propagation and to stabilize hydroperoxy radicals. (Again, the single, isolated compound curcumin was obtained from a commercial source.) Cerruti et al. used ball milling and solvent extraction to obtain antioxidant fractions from grape seeds and tomato skins. Incorporation of such carotenoid antioxidant fractions had positive effects on PP thermal stability. However, solvent extraction of antioxidant fractions and particle size reduction not only add to processing time and cost but also compromise the green, sustainable aspects of such materials. Accordingly, the search continues for natural sources of such antioxidants, aside from specific commercial materials, to better utilize the benefits and advantages available from such compounds.